1. Field of the Invention
This invention relates to a pulse tube heat engine which makes it possible to provide a simply structured, highly efficient, highly reliable and low-cost refrigerator or prime mover, wherein a pulse tube, which is the main device used in the adiabatic process of a pulse tube refrigerator, is introduced in a Stirling-cycle engine to construct a thermal cycle (a pseudo-Stirling cycle) comprising, in terms of theoretical operation, two isovolumetric processes and two adiabatic processes, whereby an expansion piston or a displacer, reciprocated at low or high temperature and heretofore essential in refrigerators or prime movers of a Stirling engine, is no loner necessary.
2. Description of the Prior Art
A Stirling cycle comprising two isothermal and isovolumetric processes is a closed-cycle apparatus which uses a working fluid (helium, argon, hydrogen, etc.) and has been developed as an external-combustion engine or refrigerator. A drawback encountered in refrigerators which use this Stirling cycle is that mechanical vibration, which is produced by reciprocation of a low-temperature, comparatively long expansion piston, is transmitted to a cold head and causes a sensor or the like to generate noise. Another problem is that contact between the outer peripheral surface of the comparatively long expansion piston and the inner peripheral surface of a cylinder produces abrasion dust that contaminates the working fluid and a regenerator. This leads to malfunctions and a decline in the performance of the refrigerator.
In order to eliminate these disadvantages of refrigerators which employ the Stirling cycle, a pulse tube refrigerator was disclosed in Low-Temperature Engineering, Vol. 26, No. 2 (1991) by Tatsuo Inoue. In this system, a radiator, a regenerator, a cold head, a pulse-tube and an orifice are serially connected between a compression space and a buffer tank to produce low temperatures using a working gas such as helium as the medium.
A pulse tube refrigerator was first proposed by W. E. Gifford in 1963. This low-temperature generating system features simply arranged component parts and, since it does not possess moving parts in its low-temperature section, there is no mechanical vibration in the heat absorber (also referred to as a cold head). For these reasons, expectations were high that it would find practical use as a highly reliable refrigerator. However, since the low-temperature generating system employs an operating principle based upon the characteristic of the non-equilibrium state of a working fluid, it is difficult to derive equations in the actual operating state and analyze the operating cycle. In addition, though the technical paper has been published from thermoacoustic and other viewpoints, there are many approximations of conditions and the principle of operation has not been established theoretically. Moreover, though efficiency is low in actual practice, it has proven that low-temperature generation is possible.
Though the principle of operation will not be touched upon here, it is clear that a simply shaped pulse tube, which is a hollow cylindrical tube made of metal or a composite material, is the main element among the component parts of the cycle, and that this tube bears the burden of the adiabatic process. In the operation of the cycle, it is believed that low temperatures are generated owing to a shift in the phase of a pressure change within the pulse tube when a fluid travels within a compression space and buffer tank.
The merit of this system is that even though operation as a prime mover is impossible solely with this engine arrangement, low temperatures can be generated without using an expansion piston reciprocated at low temperature.
This invention is concerned with a novel Stirling-cycle heat engine in which the above-mentioned pulse tube is introduced in the component parts of the Stirling cycle, described later.
The Stirling cycle is an ideal cycle theoretically comprising two isothermal processes and two isovolumetric processes. In an actual working engine, the engine is of the closed-cycle type in which helium or hydrogen is used as the working fluid (hereinafter referred to simply as the "fluid", other examples of which are neon, argon, nitrogen, air or mixed gases). In operation as a refrigerator, efficiency is higher than that of all other refrigeration cycles. Even in operation as a prime mover, it is known that vibratory noise is lower and efficiency higher in comparison with other engines.
In the meantime, a structural feature of the pulse tube refrigerator is use of a cylindrical pulse tube consisting of a metal or ceramic or a composite material thereof. During a refrigerating operation, this pulse tube exhibits a comparatively large temperature gradient and bears the burden of the adiabatic effect. However, it is well known that a refrigerator using a pulse tube is not always efficient.
Use as a refrigerator will be described with reference to FIG. 1, which shows the structure of a kinematic Stirling cycle, and FIG. 2, which illustrates P-V and T-S curves.
As illustrated in FIG. 1, a compression space 1 is connected to a crankshaft 2 driven by a motor, which is not shown. The volume of the compression space 1 is capable of being varied in a compression cylinder 4 by a connecting rod 12 and a reciprocating compression piston 3. A radiator 5, a regenerator 6 and a heat absorber 7 (in case of a prime mover, this is also referred to as a high-temperature heat exchanger or heater raised to a temperature of 900 to 1000 K as by a flame) are connected between the compression space 1 and an expansion space 10, which is defined by an expansion cylinder 8 and an expansion piston 9. In the compression space 1, a phase difference in the varying volume is advanced while maintaining a constant phase-angle difference within a range of 70.degree. to 110.degree. (the optimum phase difference is approximately 90.degree.). As for the principle of operation, theoretically the fluid in the compression space 1 is compressed isothermally while giving off heat in the radiator 5 (this is an isothermal compression process, indicated at a-b.sub. 1 in FIG. 2). Next, the compression piston 3 moves toward top dead center, as a result of which the fluid is cooled to 30 K (-243.degree. C.) by the regenerating material of the regenerator 6. The 1 cooled fluid enters the heat absorber 7 and then the expansion chamber 10 at a fixed volume (this is an isovolumetric process, indicated at b.sub.1 -c). Next, since the fluid performs the work of urging the expansion piston 9, it is recovered as effort by the crank 2 via the connecting rod 12. (This is an isothermal expansion process, indicated at c-d.sub.1, in which the foregoing occurs while heat is being absorbed from the object to be cooled, i.e., while the object is being cooled, by the heat absorber 7.) Finally, the fluid which has performed the work of expansion and resides in the expansion space 10 that is presently of maximum volume is forcibly returned to the compression space 1 from the regenerator 6 and radiator 5 as the expansion piston 9 is moved from bottom dead center to top dead center (this is an isovolumetric process, indicated at d.sub.1 -a), This ends one cycle. In FIG. 1, numeral 11 denotes a piston ring.
A disadvantage of this refrigerator (and of the prime mover as well) is that the expansion piston 9 contacts the expansion cylinder 8 and also resonates owing to the reciprocating motion of the expansion piston, which is comparatively long (35-45 cm, inclusive of a guide piston, not shown, in a case where there is one expansion space and the refrigeration output is 200 W at 80 K). As a result, mechanical vibration is produced, and this has a deleterious effect upon the object to be cooled by the heat absorber 7. For example, if this vibration is transmitted to an electronic sensor, the sensor will produce noise. Though there are displacer-type Stirling engines, inclusive of refrigerators and prime movers, in which mechanical vibration is reduced by arranging it so that the expansion piston 9 performs no work, dimensional precision deteriorates owing to large changes in temperature. Consequently, even if the comparatively long displacer, which is subjected to high or extremely low temperatures during use, is fabricated to have a high mechanical precision, contact accidents frequently occur during reciprocation. As a result, mechanical vibration is produced, and dust and gases caused by the breakdown thereof are produced owing to the contact wear of the displacer. The fluid thus becomes contaminated, leading to a decline in performance. Furthermore, the regenerator 6, which comprises innumerable small balls or a wire mesh, can become clogged owing to the dust or the mixture of impure gases and fluid (in a refrigerator, condensation and solidification of gases having a high boiling point can occur). Moreover, manufacturing costs are very high for the expansion pistons or displacers, which require a high manufacturing precision, for the finishing of the inner wall surface of the relevant cylinders, and for the manufacturing cost of the drive mechanism. As a result, use of a comparatively long expansion cylinder or displacer leads to a decline in the reliability of the Stirling engine.